Anytime it's of provable benefit to the person being protected? I'm a utilitarian, so to me everything is merely a means to the end of human happiness. This includes liberty.
I think purely viewing it as a question of liberty and not getting humans what they want has problems. The harm principle is just trying to work around the thorny issue of "The liberty to take away the liberty of others."
Why are parents better at deciding whats better for children than anyone else? I attack your premise here.
And now you ask the essential question.
Because the view of humans as overly rational creatures and children as stupid instead of merely ignorant. Humans do dumb things, children and adult alike are equal stupid at times. We watch out for each other because were social animals, and that at least is the idea behind these types of laws. Not to wield social power like a club, but as a promise to each other "hey if you do something stupid, I'm here to help."
The idea, It most certainly is wielded like a club in the current system. But the current system corrupts a lot of good ideas.


